Modern refineries employ many upgrading units such as fluid catalytic cracking (FCC), hydrocracking (HCR), alkylation, and paraffin isomerization. As a result, these refineries produce a significant amount of isopentane. Historically, isopentane was a desirable blending component for gasoline having a high octane (92 RON), although it exhibited high volatility (20.4 Reid vapor pressure (RVP)). As environmental laws began to place more stringent restrictions on gasoline volatility, the use of isopentane in gasoline was limited because of its high volatility. As a consequence, the problem of finding uses for by-product isopentane became serious, especially during the hot summer season. Moreover, as more gasoline compositions contain ethanol instead of MTBE as their oxygenate component, more isopentane had to be kept out of the gasoline pool in order to meet the gasoline volatility specification. So, the gasoline volatility issue became even more serious, further limiting the usefulness of isopentane as a gasoline blending component.
An alkylation process, which is disclosed in U.S. Patent Application Publication 2006/0131209, was developed that is capable of converting the undesirable, excess isopentane into desirable and much more valuable low-RVP gasoline blending components. The contents of U.S. Patent Application Publication 2006/0131209 are incorporated by reference herein. This alkylation process involves contacting isoparaffins, preferably isopentane, with olefins, preferably ethylene, in the presence of an ionic liquid catalyst to produce the low-RVP gasoline blending components. This process eliminates the need to store or otherwise use isopentane and eliminates concerns associated with such storage and usage. Furthermore, the ionic liquid catalyst can also be used with conventional alkylation feed components (e.g. isobutane, propylene, butene, and pentene).
The ionic liquid catalyst distinguishes this novel alkylation process from conventional processes for converting light paraffins and light olefins to more lucrative products. Conventional processes include the alkylation of paraffins with olefins, and polymerization of olefins. For example, one of the most extensively used processes in the field is the alkylation of isobutane with C3-C5 olefins to make gasoline cuts with high octane number. However, this and all conventional processes employ sulfuric acid and hydrofluoric acid catalysts.
Numerous disadvantages are associated with sulfuric acid and hydrofluoric acid catalysts. Extremely large amounts of acid are necessary to initially fill the reactor. The sulfuric acid plant also requires a huge amount of daily withdrawl of spent acid for off-site regeneration. Then the spent sulfuric acid must be incinerated to recover SO2/SO3 and fresh acid is prepared. While an HF alkylation plant has on-site regeneration capability and daily make-up of HF is orders of magnitude less, HF forms aerosol. Aerosol formation presents a potentially significant environmental risk and makes the HF alkylation process less safe than the H2SO4 alkylation process. Modern HF processes often require additional safety measures such as water spray and catalyst additive for aerosol reduction to minimize the potential hazards. The ionic liquid catalyst alkylation process fulfills the need for safer and more environmentally-friendly catalyst systems.
Benefits of the ionic liquid catalyst alkylation process include the following:
(1) substantial reduction in capital expenditure as compared to sulfuric acid and hydrofluoric acid alkylation plants;
(2) substantial reduction in operating expenditures as compared to sulfuric acid alkylation plants;
(3) substantial reduction in catalyst inventory volume (potentially by 90%);
(4) a substantially reduced catalyst make-up rate (potentially by 98% compared to sulfuric acid plants);
(5) a higher gasoline yield;
(6) comparable or better product quality (Octane number, RVP, T50);
(7) significant environment, health and safety advantages;
(8) expansion of alkylation feeds to include isopentane and ethylene; and
(9) higher activity and selectivity of the catalyst.
Ionic liquid catalysts specifically useful in the alkylation process described in U.S. Patent Application Publication 2006/0131209 are disclosed in U.S. Patent Application Publication 2006/0135839, which is also incorporated by reference herein. Such catalysts are chloroaluminate liquid catalysts comprising an alkyl substituted pyridium halide or an alkyl substituted imidazolium halide of the general formulas A and B, respectively. Such catalysts further include chloroaluminate liquid catalysts comprising a hydrocarbyl substituted pyridium halide or an hydrocarbyl substituted imidazolium halide of the general formulas A and B, respectively.
where R═H, methyl, ethyl, propyl, butyl, pentyl or hexyl group and X is a halide and preferably a chloride, and R1 and R2═H, methyl, ethyl, propyl, butyl, pentyl, or hexyl group and where R1 and R2 may or may not be the same. Preferred catalysts include 1-butyl-4-methyl-pyridinium chloroaluminate (BMP), 1-butyl-pyridinium chloroaluminate (BP), 1-butyl-3-methyl-imidazolium chloroaluminate (BMIM) and 1-H-pyridinium chloroaluminate (HP).
However, the ionic liquid catalyst has unique properties, which requires that the ionic liquid catalyst alkylation process be further developed and modified to achieve superior gasoline blending component products, improved process operability and reliability, reduced operating costs, etc. More particularly, the ionic liquid catalyst alkylation process requires uniform mixing of the hydrocarbons and catalyst, sufficient interfacial contact between the hydrocarbons and catalyst, good temperature and pressure control, and a high isoparaffin to olefin (I/O) ratio. In addition, alkylation by means of the ionic liquid catalyst is an exothermic reaction requiring the removal of heat generated.
An alkylation process disclosed in U.S. Pat. No. 5,347,064 (Child et al.) and an ExxonMobil auto refrigeration alkylation process published at page 243 of the third edition of Petroleum Refining—Technology and Economics by James Gary and Glenn Handwerk offer some attempts at improvements in general alkylation reactions, although the reactions are sulfuric acid alkylations.
The Child et al. process separates an olefin feed stream comprising at least three olefins into intermediate streams enriched in propene, 1-butene, and 2-butenes, respectively. Then a first intermediate stream enriched in propene contacts at least one isoparaffin (e.g. isobutane) in a first reaction zone at a reaction temperature specific to propene to produce a first alkylate product, a second intermediate stream enriched in 1-butene contacts the at least one isoparaffin in a second reaction zone at a reaction temperature specific to 1-butene to produce a second alkylate product, and a third intermediate stream enriched in 2-butenes contacts the at least one isoparaffin in a third reaction zone at a reaction temperature specific to 2-butenes to produce a third alkylate product. Segregation of the olefin components prior to alkylation improves alkylate quality by increasing the ratio of triemethylpentanes to dimethylhexanes in the alkylate product.
The ExxonMobil process mixes an olefin feed and a recycled isobutane stream to form a combined stream and then divides the combined stream into several streams that enter a continuous-stirred tank reactor at various points along the horizontal length of the reactor. The acid used to catalyze the reaction between the olefin feed and isobutane enters the reactor at one point only.
While the Child et al. and ExxonMobil process result in certain benefits, there still exists a need for an improved alkylation process for converting isoparaffins and olefins in the presence of an ionic liquid catalyst.